Project Summary: This proposal targets a critical challenge in the translation of AAV-mediated gene therapy, the widespread pre-existing ?AAV antibodies (Abs) in humans. Pre-existing ?AAV-Abs block AAV vectors from entering and transducing target cells. Currently, only ?AAV-Abs-negative individuals are eligible for AAV-gene therapy treatment, because no effective approach is available to overcome this challenge. Our recently published studies demonstrated that broad immune targeting is required for effective Ab-depletion by immune suppression (IS) and identified an effective IS Ab-depletion approach in a mouse model, using combination of rapamycin and prednisolone. However, this IS regimen requires up to 8-12-week-long daily administration. The IgG degrading enzymes of Streptococci (IdeS) are cysteine proteases that specifically cleaves IgG molecules. Numerous studies have demonstrated rapid and effective IgG degradation by IdeS in animals and in humans, supporting the therapeutic potential of IdeS. IdeS has a short half-life of 4.92.8hr with no demonstrated dose limiting toxicity. We therefore hypothesize that IV IdeS administration may offer a novel tool to overcome the pre-existing ?AAV-IgG for rAAV-mediated gene therapy. This proposal is to develop a novel effective ab-depletion approach for the effective translation of rAAV gene therapy to treat diseases in clinic, using a mouse model of MPS IIIB, a devastating neuropathic lysosomal storage disease, for which not treatment is currently available. Taking advantage of demonstrated rapid effective Ab removal by IdeS, this proposal will lead to the development of a novel approach for effective depletion of pre-existing ?AAV-Abs, which may make all patients in need eligible to AAV gene therapy, and viral gene therapy in general. We therefore believe that IdeS may offer the answer to the challenge posed by pre-existing Abs to the translation of gene therapy products using AAV and other viral vectors.